Hedonic and motivational responses to food reward are unchanged in rats with neuropathic pain

Abstract

Rewards influence responses to acute painful stimuli, but the relationship of chronic pain to hedonic or motivational aspects of reward is not well understood. Here, we independently evaluated hedonic qualities of sweet or bitter tastants as well as motivation to seek food reward in rats with experimental neuropathic pain induced by L5/6 spinal nerve ligation (SNL). Hedonic response was measured by implantation of intraoral catheters to allow passive delivery of liquid solutions and “liking/disliking” responses were scored according to a facial reactivity scale. SNL rats did not differ from controls in either “liking” or “disliking” reactions to intraoral sucrose or quinine, respectively, at post-surgery day 21, suggesting no differences in perceived hedonic value of sweet or bitter tastants. To assess possible motivational deficits during acute and chronic pain, we employed fixed- and progressive-ratio response paradigms of sucrose pellet presentation in rats with transient inflammatory or chronic neuropathic pain. Assessment of response acquisition and break points under the progressive ratio schedule revealed no differences between sham and SNL rats for up to 120 days post-injury. However, rats with inflammation showed decrements in lever pressing and break points on post-CFA days 1 and 2 that normalized by day 4, consistent with transient ongoing pain. Thus, while acute, ongoing inflammatory pain may transiently reduce reward motivation, we did not detect influences of chronic neuropathic pain on hedonic or motivational responses to food rewards. Adaptations that allow normal reward responding to food regardless of chronic pain may be of evolutionary benefit to promote survival.

Neuropathic pain

“…These animal models exhibit hypersensitivity to external stimuli, commonly to mechanical stimuli as assessed with von Frey hairs (for measuring the tactile sensitivity), but may also include hypersensitivity to thermal stimuli (especially cold). Higher-level outcome measurements that are suggestive of reward from pain relief and reflective of the spontaneous pain experienced by patients have recently been introduced in the array of animal models of neuropathic pain 199 . Models of diabetic neuropathy have also been affected by the ill health of the animals, but this aspect is starting to be addressed in the most recent studies 38 .…”

Reward, motivation, and emotion of pain and its relief

“…However, there are conflicting reports on the effects of pain on natural food rewards, including sucrose preference, with some studies demonstrating decreased sucrose consumption [1; 48; 50; 81; 93; 99] while others show no change [66; 76; 94]. A study by Schwartz and colleagues used mice with sciatic nerve injury-induced neuropathic pain or CFA-induced inflammation and demonstrated that even though injured animals showed no deficits in sucrose preference, they displayed decreased motivation for food reward in a progressive ratio operant responding task [76].…”

“…We used a facial reactivity score in rats to investigate the influence of chronic neuropathic pain on hedonic responses to food rewards independently from motivation [66]. Sweet or bitter liquid solutions were passively delivered via intraoral catheters to rats 21 days after spinal nerve ligation or sham surgery and “liking/disliking” responses were scored according to a facial reactivity scale.…”

“…Possible motivational deficits during acute and chronic pain was investigated using fixed- and progressive-ratio response paradigms of sucrose pellet presentation in rats with transient inflammatory or chronic neuropathic pain [66]. Assessment of response acquisition and break points under the progressive ratio schedule revealed no differences between sham and SNL rats for up to 120 days post-injury.…”

“…However, rats with inflammation showed decrements in lever pressing and break points on post-CFA days 1 and 2 that normalized by day 4, consistent with transient ongoing pain. Thus, while acute, ongoing inflammatory pain may transiently reduce reward motivation, influences of chronic neuropathic pain on hedonic or motivational responses to food rewards could not be detected [66]. Whether, and how, chronic pain may influence the value of other natural rewards remains to be determined.…”